The disclosure of Japanese Patent Application No. 2001-345936 filed on Nov. 12, 2002, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a control device and a control method for a secondary battery and, more particularly, to a discharge control of the secondary battery.
2. Description of Related Art
Electric vehicles, hybrid vehicles, and the like are equipped with a secondary battery. The hybrid vehicle is powered by, in addition to an engine, a motor that is driven by electricity supplied by the secondary battery. In addition, the hybrid vehicle has the motor function as a generator during braking, thereby transforming the kinetic energy of the vehicle into electric energy for recharging (regenerating) the secondary battery. The state of charge, i.e., SOC, of the secondary battery is therefore controlled in the range between 0% and 100%, for example, approximately 60%, so that the battery is in a state ready to receive regenerative electricity and, at the same time, the motor can be supplied with electricity immediately when required.
Depending on vehicle running conditions, however, the secondary battery may discharge a large amount of electricity, causing the SOC to become lower than a required level. Since deterioration of the secondary battery is accelerated when the battery discharges to a level of near 0% of SOC, it is necessary to stop the battery from discharging before the SOC reaches that critical level.
Japanese Patent Laid-Open Publication No. 11-122840, for example, discloses a secondary battery control device that detects a voltage of the secondary battery and, when the detected voltage becomes a predetermined threshold value or below, stops the secondary battery from discharging, thereby preventing the battery from deteriorating.
FIG. 4 shows a configuration block diagram of the secondary battery control device. A secondary battery 50 is divided into a plurality of blocks 50-1 to 50-n, each being connected in series with each other to form the secondary battery 50. Each of voltage sensors 70-1 to 70-n is connected to each of these blocks 50-1 to 50-n to monitor the voltage of the secondary battery 50. A current sensor 72 is provided in a current path of the secondary battery 50. In addition, a temperature sensor 74 is provided for detecting the temperature of the secondary battery 50. The values detected by the voltage sensors 70-1 to 70-n, the current sensor 72, and the temperature sensor 74 are supplied to a battery ECU 68. Furthermore, a relay 76 is provided in an electric power line connected to the secondary battery 50. The relay 76 functions to disconnect the secondary battery from an inverter that drives a motor which serves as a load.
The battery ECU 68 is provided with, as functional blocks thereof, a detecting portion 80, a determining portion 82, a memory 84, and a relay control portion 86. The detecting portion 80 supplies the voltage values, the current value, and the battery temperature value detected, respectively, by the voltage sensors 70-1 to 70-n, the current sensor 72, and the temperature sensor 74 to the determining portion 82 as digital data. Based on the detected battery temperature, the determining portion 82 reads out a voltage threshold value corresponding to the temperature from the memory 84. The determining portion 82 then compares the voltage threshold value with each of the detected voltage values of blocks 50-1 to 50-n of the secondary battery 50. If any one of these detected voltage values is the threshold value or lower, that is, if the lowest voltage value of these detected voltage values of the blocks is the threshold value or lower, the determining portion 82 supplies the result of this comparison to the relay control portion 86. Based on this result, the relay control portion 86 opens (turns OFF) the relay 76, thereby disconnecting the secondary battery 50 from the inverter. This stops the secondary battery 50 from discharging any further, thus preventing it from deteriorating.
There is known another technique that is not concerned with turning ON or OFF the relay 76 as described in the foregoing paragraphs. Instead, if the battery ECU 68 determines that the voltage of the secondary battery 50 is the threshold value or lower, it sends that result to an ECU for controlling the entire system which may be, for example, a hybrid (HV) ECU that controls the engine and the inverter. The HV ECU then controls the inverter so as to restrict an output from the motor, thereby controlling a voltage drop in the secondary battery 50. While turning OFF the relay stops the operation of the motor all of a sudden which may cause driveability degradation, controlling the inverter to reduce power consumption of the motor can suppress an effect on driveability and at the same time can prevent overdischarge of the secondary battery 50.
In addition to controlling the inverter based on the result of the determination supplied by the battery ECU 68, the HV ECU executes, preferentially over the inverter control, an engine control based on an acceleration operation, brake operation, or the like and a motor control based on the SOC of the secondary battery 50. Processing of the inverter control based on the result of the determination supplied by the battery ECU 68 is given a low order of priority. This could present a new problem, in which the voltage of the secondary battery 50 becomes lower than the threshold value as a result of a control lag of the inverter. If, on the other hand, a setting is made to allow a sufficient margin for the lowest voltage so as not to let the voltage become lower than the threshold value, electricity charged in the secondary battery can be used only insufficiently. The same holds true with a case in which consideration is given to prevent an excessive rise in voltage when recharging the secondary battery.
Assume, for example, that the battery ECU 68 operates at a rate of 100 ms, while the HV ECU operates at 8 ms. The battery ECU 68 detects a voltage at 100 ms, i.e., receives a signal of the detected voltage, compares it against the threshold value, and supplies the result of the comparison to the HV ECU. The HV ECU does not, however, process the data of the detected voltage value at 8 ms. Rather, HV ECU operates to control the inverter after the lapse of a period of 300 ms or more, since processing of the data is given a low order of priority. As motor keeps operating during this period of time, there arises a problem of a further voltage drop in the secondary battery 50.
It is an object of the invention to provide a secondary battery control device that is capable of preventing a voltage drop in a secondary battery by promptly processing data of a detected voltage value and thereby suppressing deterioration of the secondary battery.
A first aspect of the invention relates to a secondary battery control device. The secondary battery control device is provided with: a voltage detector that detects a voltage of the secondary battery; a first controller that receives at every predetermined period of time a value representing the voltage detected by the voltage detector and controls the secondary battery; and a second controller that receives the detected voltage from the first controller, determines within a period of time shorter than the predetermined period of time whether or not the received voltage is greater than a predetermined threshold value, and, based on the result of determination made, controls recharge or discharge of the secondary battery.
As explained above, in the secondary battery control device according to the first aspect of the invention, the first controller receives the voltage value of the secondary battery at every predetermined period of time, and the second controller controls recharge or discharge of the secondary battery at every period of time shorter than the predetermined period of time. This arrangement makes it possible to control the secondary battery promptly. Therefore, the secondary battery control device can control the amount of discharge or the amount of recharge of the secondary battery.
A second aspect of the invention relates to a secondary battery control method. The method includes the steps of detecting a voltage of the secondary battery at every predetermined period of time; determining within a period of time shorter than the predetermined period of time whether or not the detected voltage value is greater than a predetermined threshold value; and controlling recharge or discharge of the secondary battery based on the result of the determination made.